A stability factor is one of the significant factors affecting the vehicle behavior. Provided that the vehicle is turning steadily without accelerating and decelerating, the stability factor can be expressed using an inertial mass, a wheel base, cornering powers (i.e., cornering forces) of front and rear wheels, a distance between a gravity center of the vehicle and a front axle, a distance between the gravity center of the vehicle and a rear axle and etc. Otherwise, the stability factor can be obtained based on an actual steering angle, a wheel base, a lateral acceleration and a vehicle speed. Such formula may be modified to calculate the stability factor of the case in which the vehicle is turning while accelerating or decelerating. Specifically, the stability factor of this case may be calculated using a quadratic expression in which a term multiplying a longitudinal acceleration by a coefficient and a term multiplying the square of longitudinal acceleration by a coefficient are added to the stability factor of the case without acceleration and deceleration. To this end, specifically, the coefficient relating to compliance between a load shift resulting from acceleration or deceleration and a change in a toe angle, and a coefficient relating to a cornering characteristic of tires subjected to a driving force and a braking force, are used in the above-mentioned expression.
For example, a turning radius, and a yaw rate during turning are changed depending on a value of the stability factor. Thus, the stability factor is a major parameter (i.e., a physical quantity) having a great influence on the steering characteristics of automobiles. Basically, the stability factor is governed by a structure of the vehicle, characteristics of tires etc. However, cornering powers of front and rear wheels may be changed depending on loads applied to the front and rear wheels, deterioration of the wheels with age and so on. In addition, in the formula for calculating the stability factor thus modified, the coefficients used in the linear term and the quadratic term of the longitudinal acceleration may not always correspond to designed value.
Over the years, attempts have been made to correct the stability factor for the purpose of controlling vehicle behavior. For example, Japanese Patent Laid-Open No. 2005-256366 discloses a control system configured to stabilize vehicle body attitude by suppressing the effect of driver operation disturbance or road surface disturbance. The control system taught by Japanese Patent Laid-Open No. 2005-256366 have been conceived noting a fact that the stability factor fluctuates due to fluctuation of ground loads on the front and rear wheels. Therefore, the control system taught by Japanese Patent Laid-Open No. 2005-256366 is configured to correct an axle torque in a manner such that the difference between the products of front wheel and rear wheel cornering powers and distances between the center of gravity of the vehicle and each front and rear wheels (that is, moments derived from the cornering powers of the front and rear wheels) follow the target value.
Meanwhile, Japanese Patent Laid-Open No. 2008-275344 discloses a device for calibrating the zero point of a yaw rate sensor for detecting a yaw rate, that is, a turning condition of the vehicle. Specifically, the device taught by Japanese Patent Laid-Open No. 2008-275344 is configured to calibrate the zero point of the yaw rate sensor by detecting a turning condition of the vehicle using a sensor other than the yaw rate sensor, and comparing the detection value with a value detected by the yaw rate sensor.
That is, the control system taught by Japanese Patent Laid-Open No. 2005-256366 is configured to control the torque in a manner such that the actual stability factor follows the target value. For this purpose, the actual stability factor is obtained based on a detection value of the sensor for detecting a behavior of the vehicle such as the yaw rate sensor. However, if the detection value of the sensor has an accidental error, the stability factor will not represent the actual vehicle behavior. In this case, therefore, the torque may not be controlled properly to deteriorate a drive feeling.
As described, the device taught by Japanese Patent Laid-Open No. 2008-275344 is configured to calibrate the zero point of the yaw rate sensor so that an accidental error of the sensor value is reduced. However, according to the teachings of Japanese Patent Laid-Open No. 2008-275344, another sensor such as a radar is required to detect a turning condition of the vehicle in addition to the yaw rate sensor, and this complicates a structure of the device and enlarges the device. Further, the calibration has to be influenced by a detection accuracy of another sensor. Thus, the device has to be improved in several respects.